1. Field of the Invention
The present invention relates to a semiconductor processing system, and more particularly, to a gas injection apparatus for supplying a reactive gas for processing a semiconductor substrate into a reaction chamber.
2. Description of the Related Art
Semiconductor processing systems such as plasma processing systems or magnetron sputtering systems have been mostly used in a micromachining process performed on a semiconductor substrate for manufacturing a semiconductor device or flat display panel. For example, plasma enhanced chemical vapor deposition (PECVD) systems or high-density plasma CVD (HDP-CVD) systems have been mostly used to deposit a material layer on a substrate by CVD. Magnetron sputtering systems have been widely used to deposit a material layer on a substrate by physical vapor deposition (PVD).
Semiconductor processing systems are being developed that best suit various process characteristics for semiconductor processing. In particular, as substrate diameter increases, recent developments in semiconductor processing systems have focused on coping with large-sized substrates to achieve improved yield. That is, as wafer sizes change from 200 mm to 300 mm, improvement in uniformity over a large wafer is of particular concern to wafer processing. To achieve the desired uniformity, an important factor is the uniform distribution of reactive gas across a reaction chamber when supplying it to the reaction chamber through a gas injection apparatus for a semiconductor processing system.
Accordingly, to achieve the uniformity in gas distribution through a gas injection apparatus, many different types of gas injection apparatuses have been developed. One such gas injection apparatus has a plurality of nozzles positioned at a plurality of levels in a direction perpendicular to a substrate. A gas distribution nozzle positioned at a higher level extends further towards the center of a reaction chamber than nozzles at a lower level. This gas injection apparatus may be effective in achieving the gas distribution uniformity, but is likely to prohibit ion flux flowing to the substrate due to the portions of nozzles extending toward the interior of the reaction chamber.
Another type of gas injection apparatus is a showerhead type gas injection apparatus. Here, gas is supplied to a showerhead through baffles and holes in order to provide a uniform gas pressure distribution across the backside of the showerhead. Another example of a showerhead type gas injection apparatus varies the sizes of holes in a baffle plate to achieve an even gas pressure distribution at the backside of a showerhead. However, it is very difficult to optimize these systems for a wide range of gas pressures and flow rates. Another drawback is that the showerhead type gas injection apparatus is suitable only for a parallel plate plasma reactor and cannot be applicable to magnetron sputtering systems. Furthermore, when used in an Electron Cyclotron Resonance (ECR) plasma reactor, the showerhead disposed at an upper portion of the reaction chamber prohibits microwave propagation.
Instead, a ring-shaped gas injection apparatus, an example of which is shown in FIG. 1, is used for ECR plasma reactors and other actual applications. The ring-shaped gas injection apparatus 10 shown in FIG. 1 has a gas channel 14 formed therein so that reactive gas can pass through the gas channel 14, a gas inlet 12 connected to the gas channel 14 at the outer circumference of the apparatus 10, and a plurality of nozzles 16 at the inner circumference of the apparatus 10. The nozzles 16 are evenly spaced along the inner circumference of the gas injection apparatus 10.
The gas injection apparatus 10 is configured to form a gas path connecting the gas inlet 12 to each of the plurality of nozzles 16 through the gas channel 14. The length of the gas path between the gas inlet 12 and each nozzle 16 varies. The difference in gas paths creates a difference in pressure of reactive gas at each nozzle due to a pressure drop after collision of reactive gas with a wall of the gas channel 14. This makes the gas flow rate through each nozzle 16 uneven.
FIG. 2 is a graph showing gas pressure and flow rate at each nozzle in the gas injection apparatus of FIG. 1. In the graph, the gas inlet 12 is labeled as number “0”, and the nozzles are labeled as the numbers “1” to “16” in the order that they are disposed along the inner circumference of the gas injection apparatus.
The graph in FIG. 2 shows the result of calculating gas pressure and flow rate when supplying O2 gas at a flow rate of 100 standard cubic centimeters per minute (sccm) under a pressure of 10 mTorr in the reaction chamber. The gas channel has a rectangular cross-section of 1×4 mm, and each nozzle has a diameter of 0.5 mm and a length of 2 mm. The gas channel is formed into a ring shape having a diameter of 241 mm.
As is evident from the graph in FIG. 2, gas pressure decreases in a direction away from the gas inlet, i.e., as the length of the gas path increases. The gas flow rate at the nozzle located closest to the gas inlet is approximately four times that at the nozzle located farthest from it. In this manner, a conventional ring-shaped gas injection apparatus makes gas pressure and flow rate at each nozzle extremely nonuniform due to differences in the lengths of gas paths connecting a gas inlet to each nozzle through a gas channel.
The design and configuration of a gas injection apparatus significantly affects the uniformity over a substrate being processed. As the size of the gas injection apparatus increases and the sectional area of the gas channel decreases, nonuniformity in gas distribution becomes larger. In order not to prohibit ion flux and microwave propagation, it is desirable to make the gas injection apparatus as thin as possible. Despite this structural requirement for a gas channel having a small sectional area, a large gas injection apparatus with a thin gas channel still has a problem with uniform pressure distribution.
Accordingly, as wafer sizes continue to increase, ensuring even gas distribution to the reaction chamber with a conventional gas injection apparatus becomes more difficult. This deteriorates uniformity across a semiconductor substrate being processed, which in turn significantly worsens the quality and yield of semiconductor devices.